1. Field of the Invention
The present invention relates to an excess current interrupting structure capable of shutting off an electric current path, which is connected to a load, with a high responsibility and a high reliability when the current path is put in the condition in which an excess current flows therein, and more particularly to an excess current interrupting structure which is formed by using as a conductive wire rod generating heat by an excess current a wire rod, for example, a fine gold wire, a copper wire and an aluminum wire in which good conductors including gold, copper and aluminum are rendered capable of being handled as wire rods of a relatively high resistance; and covering a circumference of this conductive wire rod with a resin, and which is adapted to enable the electric current path in which an excess current flows to be completely shut off.
2. Description of the Related Art
In a related electric current path connected to a load, a fuse is inserted, and, when the current path is put in the condition in which an excess current flows therein, the fuse is melted to shut off the electric current path. As known well, this fuse employs a structure of a cartridge fuse 101 shown in FIG. 16 in, for example, a load power source line in which an accident current is large, so as to protect a fuse wire rod, and a structure of a tubular fuse 102 shown in FIG. 17 in an electronic machine and tool in which an accident current is small, so as to protect the machine and tool. However, these fuses have the following problems.
(1) The cartridge fuse 101 has a complicated construction, a high degree of difficulty in manufacturing and a high price, so that it causes a large cost increase in the overload protection in a load power source line.
(2) Both the cartridge fuse 101 and the tubular cartridge 102 use a fuse holder, so that, when they are contained in a machine and a tool, a space occupied thereby increases.
Repeated discussions and experiments on an excess current interrupting structure superseding the fuses which have such problems come to propose a new excess current interrupting structure. This new excess current interrupting structure uses as a core a conductive wire rod, which generates heat by an excess current, such as, for example, a fine gold wire and a fine copper wire in which good conductors including gold, copper and aluminum are rendered capable of being handled as wire rods of a relatively high resistance, and is thereby formed into a structure in which a circumference of this conductive wire rod is coated with a resin. Therefore, the securing of the capacity of an electric current path (heat radiation to the surrounding of the path) and the fixing of a pair of electric conductors (for example, terminals) connected together by a conductive wire rod can be attained. In addition, this structure is advantageous in view of its easiness in handling, resistance to environment and insulating characteristics with respect to peripheral parts.
However, in the case of a structure in which a circumference of a conductive wire rod is coated with a resin, it should be noted that the carbonization of the resin around the conductive wire rod progresses in a stage in which the conductive wire rod comes to be melted by an excess current with this carbonized portion constituting a bypass electric current path (carbonized path). Namely, even when the conductive wire rod is melted by an excess current, the bypass electric current path formed of the carbonized portion remains, and an excess current necessarily continues to be supplied to the load. Therefore, a related structure in which a circumference of a conductive wire rod is coated with a resin lacks practicability.
The present invention has been effected with attention paid to the above-mentioned problems of the related art, and aims at providing a practical excess current interrupting structure, in which a circumference of a conductive wire rod is coated with a resin, capable of shutting off with a high responsibility and a high reliability an electric current path in which an excess current flows.
An object of this invention is to provide an excess current interrupting structure comprising a conductive wire rod which generates heat by an excess current connected between a pair of electric conductors, a circumference of the conductive wire rod being covered with a resin with a thin coated layer of the covering formed in the vicinity of a path of the conductive wire rod.
In the invention, when the conductive wire rod connected between a pair of electric conductors generates heat by an excess current to cause the temperature thereof to increase with the resin layer also generating heat at the same time to cause the temperature thereof to increase, the temperature rise in the resin layer is concentrated in the section thereof in which the thin coated portion is formed. In consequence, thermal stress and a large quantity of gas occur in a stage of carbonization of the resin in the section of the resin layer in which the thin coated portion is formed rather than in the section thereof which remains to have a thickness of the resin layer. In other words, an increase in thermal stress and a gas pressure in the section of the resin layer in which the thin coated portion is formed becomes sharper than that of the thermal stress and the gas pressure in the section thereof which remain to have a thickness of the resin layer.
Therefore, when thermal stress and a gas pressure are in the condition far from that in which the thermal stress and the gas pressure becomes a critical bursting pressure in the section which remains to have a thickness of the resin layer in a stage in which a carbonized path is formed ascribed to the carbonization of the resin around the conductive wire rod caused by a flow of an excess current through the conductive wire rod, thermal stress and a gas pressure in the section in which the thin coated portion is formed reach a critical bursting pressure, and the section bursts on a small scale. Consequently, the section in which the thin coated portion is formed is momentarily destroyed with the carbonized path as if cracks occurred in a fault, no matter whether the conductive wire rod is melted by an excess current or not. As a result, when a load current flowing in the current path between a pair of electric conductors has become an excess current, the current path in the section in which the thin coated portion is formed will be able to be shut off with a high responsibility and a high reliability owing to the non-destroyed condition of the section which remains to have a thickness of the resin layer. In addition, when the thickness of the thin coated layer of covering is regulated, a degree of freedom of design to finely regulate the shut-off time for the current path will be obtained.
In a first sub-aspect the excess current interrupting structure according to the invention, the conductive wire rod is a conductor wire of a reduced diameter comprising a metal of a high electric conductivity.
In the above first sub-aspect, a degree of freedom of design to arrange a conductive wire rod generating heat by an excess current will be obtained.
In a second sub-aspect of the excess current interrupting structure according to the invention, the conductive wire rod is plurally connected between the pair of electric conductors.
In the above second sub-aspect, it will be possible to expand a heat source region occurring when an excess current flows between the pair of electric conductors, and expand a destruction starting point. This can be effected by reducing the diameter of the electric wire rod in inverse proportion to the number thereof.
In a third sub-aspect of the excess current interrupting structure according to the invention, the pair of electric conductors are formed based on a material and volume thereof selected so that the generation of heat by an excess current is concentrated on the conductive wire rod.
In the above third sub-aspect, the conductive wire rod will generate heat with a high responsibility when an excess current flows in the current path.
In a fourth sub-aspect of the excess current interrupting structure according to the invention, the pair of electric conductors are of a shape according to surface mounting specifications.
In the above fourth sub-aspect, the electric conductors will be able to be mounted easily on a circuit board by utilizing a surface mounting technique.
In the fifth sub-aspect of the excess current interrupting structure according to the invention, the thin coated layer of covering is formed by a transfer mold using a metal mold.
In the above fifth sub-aspect, the thin coated layer of covering is formed when the conductive wire rod connected between the electric conductors is formed by a transfer mold using a metal mold, so that a production process will be simplified.
In a sixth sub-aspect of the excess current interrupting structure according to the invention, the thin coated layer of covering is provided therein with an abnormality detecting means, such as a temperature sensing substance and a physical force sensing substance.
In the above sixth sub-aspect, when the thermal destruction of the thin coated layer of covering cannot be recognized visually from the outside, the occurrence thereof will be able to be known by a change in a color tone based on the temperature sensed by the temperature sensing substance.